A unique signal detected in human brain : ScienceAlert

Scientists have identified a unique form of cell messaging that occurs in the human brain, and shows how much we still have to learn about its mysterious workings.

The discovery is an interesting clue that our brains may be even more powerful when it comes to math than we thought.

In 2020, researchers from institutes in Germany and Greece reported on a mechanism in the brain’s outer cortical cells that produces a new, “graded” signal on its own. A signal that could provide individual neurons with a different way to perform their logical functions.

By measuring electrical activity in tissue samples removed from epilepsy patients during surgery and analyzing their structure using fluorescence microscopy, the neurologists discovered that individual cells in the cortex use not only the usual sodium ions to “fire,” but also calcium.

This combination of positively charged ions produces voltage waves that have never been observed before. These waves are called calcium-mediated dendritic action potentials (dCaAPs).

Brains—especially those of the human species—are often compared to computers. The analogy has its limitations, but at some levels they perform tasks in similar ways.

Both use the power of an electrical voltage to perform various actions. In computers, it is in the form of a fairly simple flow of electrons through junctions called transistors.

In neurons, the signal is in the form of a wave of opening and closing channels that exchange charged particles, such as sodium, chloride, and potassium. This pulse of flowing ions is called an action potential.

Instead of transistors, these messages are chemically processed by neurons at the end of branches called dendrites.

“The dendrites are critical to understanding the brain because they are at the heart of what determines the computational power of individual neurons,” Humboldt University neuroscientist Matthew Larkum told Walter Beckwith at the American Association for the Advancement of Science in January 2020.

Dendrites are the traffic lights of our nervous system. If an action potential is significant enough, it can be passed on to other nerves, which can block or pass on the message.

This is the logical basis of our brains – ripples of tension that can be collectively communicated in two ways: either a AND message (if x And y are activated, the message is passed on); or a OR message (if x or y is activated, the message is passed on).

Nowhere is this more complex than in the dense, wrinkled outer part of the human central nervous system; the cerebral cortex. The deeper second and third layers are particularly thick, full of branches that carry out higher functions that we associate with sensation, thought and motor control.

The researchers looked closely at the tissues from these layers. They connected the cells to a device called a somatodendritic patch clamp. This sends active potentials back and forth through each neuron and records the signals.

“When we first saw the dendritic action potentials, there was a ‘eureka’ moment,” Larkum said.

To make sure the findings didn’t just apply to people with epilepsy, they double-checked their results in a handful of brain tumor samples.

Although the team had conducted similar experiments in rats, the signals they saw buzzing through the human cells were very different.

More importantly, when they gave the cells a sodium channel blocker called tetrodotoxin, they still found a signal. Only by blocking calcium did everything stop.

Finding an action potential mediated by calcium is interesting enough. But modeling how this sensitive new kind of signal worked in the cortex revealed a surprise.

Besides the logical AND And OR-type functions, these individual neurons could act as ‘exclusive’ OR (XOR) intersections, where a signal is only allowed if another signal is graded in a certain way.

“Traditionally, the XOR “It was thought that a network solution was required for this operation,” the researchers wrote.

More work needs to be done to see how dCaAPs behave in whole neurons and in a living system. And that’s not even to say whether this is a human thing, or whether similar mechanisms have evolved elsewhere in the animal kingdom.

Technology also draws on our own nervous systems for inspiration in developing better hardware. If we know that our own cells have a few more tricks up their sleeves, we can come up with new ways to network transistors.

How this new logical tool, compressed into a single nerve cell, translates into higher functions is a question for future researchers to answer.

This research was published in Science.

A version of this article was originally published in January 2020.

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